Significance Attenuated measles vaccine is one of the most safe and effective vaccines available. However, vaccinated persons may become silently infected when exposed to measles virus and transmit the infection to other unvaccinated persons. And 1 to 5% of vaccinated persons fail to make antibody responses after vaccination. Objectives 1) to test the hypothesis that measles virus can persist and be transmitted by immune individuals, and 2) to study the cellular immune response to measles virus in vaccine non-responders. Results A group of 9 juvenile monkeys were housed together in a large cage enclosure to allow social housing and to study measles virus transmission. Five of the monkeys had been infected with measles virus 6 months earlier and were healthy and virus isolation negative. Four monkeys had never been exposed to the virus or to vaccine. Blood samples were taken every month for one year and tested for antibody to measles virus. None of the 4 unexposed monkeys developed a skin rash or antibody to measles, thus the virus was not transmitted from healthy, measles-recovered animals. After screening for measles antibody in vaccinated monkeys in the colony, several antibody-negative, vaccine non-responders were identified. Three of these monkeys and 3 vaccine-responder monkeys were further tested. After several timed blood samples, all 6 monkeys received another dose of measles vaccine. The 3 non-responder monkeys seroconverted with measles antibody, but their serum levels dropped after 2 or 3 months to unprotective levels. Future Directions The T lymphocytes of vaccine non-responder monkeys will be tested for helper T cell function and cellular immune responses to measles virus. KEY WORDS measles, vaccine, infant health FUNDING NIH Grant RR00169, CDC Cooperative Agreement CCU913348